It is known in the prior art to employ a train vehicle control signal voltage threshold determining device to establish the presence of a train vehicle within a defined signal block of a track system, such as disclosed in U.S. Pat. No. 3,657,663 of R. S. Rhoton et al. A transistor threshold circuit is employed in relation to the forward conduction voltage of its base emitter junction for the desired signal voltage threshold determination function. A speed coded audio frequency carrier train vehicle control signal is supplied to a particular signal block for controlling the operating speed of a train vehicle present within that signal block, and in addition an occupancy detection receiver is operative with the signal block to determine when that signal block is occupied by a train vehicle in relation to the absence of the supplied speed coded signal at the input of that receiver.
It is known in the prior art to employ an optically coupled logic network such as disclosed in U.S. Pat. No. 3,826,930 of R. H. Perry. A light emitting diode is operative with a light responsive device such as a photo diode for controlling the logic operation of that network.
It is known in the prior art to employ a Zener impedance breakdown diode to sense the voltage level of an applied signal to determine the threshold level of that applied signal as disclosed in U.S. Pat. No. 3,760,192 of J. O. G. Darrow. The Zener diode level detector can be used to provide the operating potential to an oscillator whenever the applied input signal level is adequate to break down the impedance of the Zener diode.
It is desired that a failsafe train vehicle control signal voltage threshold determination function be provided wherein leakage current is no problem such as a collector to base leakage current in conjunction with the impedance of a base lead or a collector lead to develop an undesired voltage across the latter impedance for forward biasing the device and making it unreliable for voltage threshold reference purposes.